The present invention relates to an ink jet recording sheet with which recording is performed using ink and in particular, to an ink jet recording sheet which is inhibited from occurrence of undulation of the sheet caused by absorption of ink after recording and which is assured in curling resistance hardly susceptible to change in temperature or humidity.
The ink jet recording method performs recording of graphics and characters by allowing ink droplets ejected by various working principles to deposit on a recording sheet such as a paper. The ink jet recording has such favorable features that it makes high-speed recording possible, that it produces little noise, that it can easily perform multi-color recording, that there is no limitation as to kind of patterns or images, and that it requires no processing for development and fixing. Thus, the ink jet recording is rapidly becoming widespread in various fields as devices for recording various figures including kanjis (Chinese characters) and color images. Furthermore, the images formed by the multi-color ink jet recording method are not inferior to those printed by multi-color press or those obtained by color-photography. Besides, use of the ink jet recording extends to a field of full-color image recording where number of copies is not so many, since costs per copy are less than those employing the photographic process.
As for the recording sheets used for this ink jet recording, efforts have been made from the aspects of printer hardwares or ink compositions in order to use woodfree papers or coated papers used for ordinary printing or writing. However, improvements in recording sheets have come to be required increasingly in order to go side by side with developments in printer hardwares such as ever increasing speed, development of ever finer definition images of full color, and also with expanding fields of uses. That is, recording sheets are demanded to develop ever high image reproducibility, and in order to meet that demanded image be maintained high, and hue characteristics be bright and appealing, the ink applied be fixed quickly and does not bleed or spread even though a different ink is put over additionally. Moreover, ink should set quickly, dots should not spread more than needed and the circumference of dots be sharp and demarcating.
Various proposals have been made for such demand. For example, an ink-receiving layer mainly composed of silica pigments which forms a void layer is provided on the surface of a support to improve ink-absorbing property (Japanese Patent Application Kokai Nos 52-9074 and 58-72495). In order to improve ink absorption property by providing this ink-receiving layer to obtain a high printed dot density and printed dots free from spread of ink dots, Japanese Patent Application Kokai Nos. 55-51583 and 556-157 propose to add non-colloidal silica powders to the ink-receiving layer. Furthermore, noticing that color quality and clearness depend on the state of the distribution of the dye of the ink in the ink-receiving layer, it has been proposed to use a specific agent which adsorbs the dye component (Japanese Patent Application Kokai No. 55-144172).
Further, as another problem encountered after performing the ink jet recording, there is the occurrence of undulation of the sheet after printing with aqueous ink. If the recording sheet after printing has undulation, beautifulness of appearance which is organ-oleptically evaluated is deteriorated although the sheet is excellent in image reproducibility. The undulation is an irregularity generated due to expansion and contraction of wood pulp in the support caused by the permeated ink. Accordingly, to inhibit permeation of the ink into the support is a measure for avoiding occurrence of undulation. This is the same means as allowing the ink-receiving layer to absorb a large amount of ink and when amount of the ink-receiving layer is increased, peeling off (exfoliation in the form of powders) of the coat is apt to occur.
For inhibiting the permeation of an aqueous ink without increasing the amount of the ink-receiving layer, it can be considered to reduce the void capillary content in the ink-receiving layer and the support or to increase water repellency. As a result, however, permeation of ink into the ink-receiving layer or the support is delayed and the ink is in undried state and the ink overflows around dots formed by overlapping of inks or during running of the sheet on an ink jet printer, the sheet contacts with devices provided around sheet feeder and the printed portion is rubbed, resulting in stain of the printed portion called staining of background. When this stain occurs over a wide area, the non-image area is stained to damage the appearance and in addition, the adjacent dots contact with each other to cause spread of dots. As a result, clearness of the image decreases and color quality is deteriorated owing to mixing of colors and thus, image reproducibility is considerably deteriorated.
In order to avoid the occurrence of unevenness after printing, various proposals have been made and recently, it has been attempted to use a platy pigment and a binder having a glass transition temperature of 30.degree.-60.degree. C. in combination in a backcoat layer (Japanese Patent Application Kokai No. 4-298380). It is generally said that a treatment at a temperature higher than the glass transition temperature is necessary for assuring adhesiveness using an emulsion type binder and a treatment at a temperature at least about 60.degree. C. higher than the glass transition temperature is necessary for developing a complete adhesiveness (Soichi Muroi, "Polymer Latex Adhesives" published from Kobunshi Publishing Association). Thus, in order to maintain adhesiveness and inhibit peeling of the backcoat layer, it is essential to increase amount of the binder or to treat the backcoat layer at a temperature higher than the glass transition temperature, preferably at least about 60.degree. C. higher than the glass transition temperature.
However, increase of the amount of the binder causes the phenomenon of the sheets sticking to each other which is called blocking. In the ink jet recording apparatus having a mechanism where a plurality of sheets is set and printed one by one, the blocking brings about the problems that not only two or more sheets are delivered at one time, but also the sheet cannot be delivered owing to clogging of the sheets. Further problem encountered is that the ink-receiving layer is peeled and sticks to the backcoat layer because the blocking is a phenomenon of sticking of the backcoat layer and the ink-receiving layer to each other. Moreover, when adhesiveness of the backcoat layer is ensured by raising the drying temperature at the drying step after coating the backcoat layer, migration of the binder in the backcoat layer is accelerated and consequently, distribution of the binder in the backcoat layer becomes ununiform, resulting in formation of waviness already before printing.
In the ink jet recording sheet, since the ink-receiving layer, the backcoat layer and the support expand and contract by influence of temperature or humidity, the sheet is being required to have the property not to curl (curling resistance) under conditions of any temperatures and humidities with the spread of ink jet recording apparatuses. Curling of the recording sheet occurs due to the difference in expansion and contraction of the front side and the back side of the sheet with the change in temperature and humidity. For example, if the contraction of the front side is greater than that of the back side under the environment of low temperature, the recording sheet warps upwardly.
If the recording sheet lacks curling resistance, the sheet cannot be fed to or discharged from the ink jet recording apparatus owing to curling of the sheet and in addition, the problem of staining of the sheet also occurs. An attempt to ensure the curling resistance by using a polyethylene terephthalate film as a support is disclosed in Japanese Patent Application Kokai Nos.61-235184 and 62-282967. However, it is difficult to obtain curling resistance exhibited under any temperatures and humidities which has been recently required and furthermore, it is mostly impossible to obtain that effect in the recording sheet having a support mainly composed of a wood pulp and a pigment.
With recent increase in demand for visualization, the number of printed dots per unit area of the recording sheet increases in order to improve clearness and color quality. This means increase of the amount of permeating ink and as a result, undulation after printing readily occurs. Furthermore, with the spread of ink jet recording apparatus, the environment in which the recording is performed ranges from low temperature and low humidity to high temperature and high humidity and it is increasingly demanded to ensure curling resistance under conditions of any temperatures and any humidities. Furthermore, with increase in the speed of recording apparatuses, the required level for curling resistance becomes severer.